KAISERSLAUTERN RESEARCHERS DEVELOP POWERFUL MINIMOTOR

Classic motors convert a form of energy such as heat into mechanical work. Can these laws also be transferred to a miniature machine that consists of only a single cesium atom and could thus work more efficiently? A team of researchers at the Technical University of Kaiserslautern, led by physics professor Dr. Artur Widera, has provided the proof. Moreover, with the help of a trick from the quantum toolbox, the scientists were able to operate the machine stably despite the fluctuations that are omnipresent in the quantum world. The associated research has now been published in the journal Nature Communications.

A classically powered engine follows the laws of thermodynamics. For example, gasoline is ignited and thermal energy is converted into kinetic energy by the pistons. Widera's research group, in collaboration with Prof. Dr. Eric Lutz of the University of Stuttgart, has transferred these basic principles to the quantum world, addressing fundamental questions of thermodynamics in quantum mechanics.

But how can such a quantum heat machine be built in the first place? For this purpose, the researchers chose a special experimental setup: A gas of rubidium atoms serves as the medium, which - in order to exclude thermal fluctuations - was cooled down to almost absolute zero. The fuel in the system is the spin of the rubidium atoms, i.e. their intrinsic angular momentum. The miniature machines are made of individual cesium atoms; the necessary heat exchange occurs when the cesium and rubidium atoms collide.

"The spin can be in two directions, up or down, which in our system represents hot and cold and thus the heat difference," explains Jens Nettersheim, a doctoral student and first author of the study. "When the so-called spin-exchange collisions take place, the rotational motions of the colliding cesium and rubidium atom tilt in the other direction. At the ultracold temperatures, we can control the direction of the spin change in individual collisions. We have replaced the motion of the piston that converts the energy in the system with a changing magnetic field." Using these analogies to heat exchange and piston motion, the physicists have succeeded in realizing an Otto cycle in the quantum world.

In doing so, the research team has overcome a challenge that was previously considered irrefutable: "In general, the properties or states of quantum particles cannot be determined unambiguously," Widera explains. "That is, we can measure them, but we can never predict with certainty the result of a single measurement. I can only determine the probability with which the observed properties occur." It is precisely these "fuzziness" or fluctuations in measurement results that have so far led the scientific community to doubt that a quantum heat engine can deliver constant power with high efficiency at all. "I would like to fundamentally rule out the possibility of an engine fluctuating uncontrollably between different power levels," Widera said.

During the spin-exchange shocks, these fluctuations occurred as well, but the research team found, "Over time, the spin of the cesium atoms saturates," Widera says. "In other words, they persist in one state after a certain time, so fluctuations are controllable. Compared to 'classical' thermal machines, the atoms reach a higher state of excitation in the process. This is the key to the efficient operation of a quantum thermal machine.  In addition to the advantage of suppressed fluctuations, this quantum trick allows quantum machines to convert even more energy in one cycle than is thermodynamically possible with hot and cold baths."

The quantum heat engine developed by the researchers runs reliably while unleashing consistently high power, and does so with very high efficiency. Widera's group has thus succeeded in successfully bringing thermodynamics together with the quantum world in experiments and, together with theoretical support from Prof. Lutz, has further opened the door to the application of quantum thermodynamics.

The study has appeared in the prestigious journal Nature Communications:
“A quantum heat engine driven by atomic collisions”
rdcu.be/ch9OV
doi.org/10.1038/s41467-021-22222-z

Questions to:
Prof. Dr. Artur Widera
Department of Individual Quantum Systems
phone: 0631 205-4130
E-Mail: widera(at)physik.uni-kl.de macos/deepLFree.translatedWithDeepL.text